Thanks for that. Yes, I guess I was too zoomed into the issue to grasp the wider picture…. 😊
That said, it seems from what I’ve read that the file /wp-admin/customize.php contains all the current contents of the “missing” menu option – “Additional CSS WP settings” UNLESS the theme has been changed.
It appears that a change of theme empties the CSS data from that file. *** Which is probably why Xtreme warns not to use it:
Additional CSS – WP Setting : Help
“NOT RECOMMENDED! The standard WordPress “Additional CSS” rules defined here will NOT be saved with your Weaver Xtreme settings. If needed for some specialized circumstances, the “Additional CSS” rules are preserved across all Weaver Xtreme subthemes and settings versions on an individual site, but we suggest always using the “Global Custom CSS” option above.”
*** Just checked my main site customize.php and this all seems to be nonsense — other than the Xtreme warning….
So I have just moved all my data from “Additional CSS – WP Settings” to “Global Custom CSS” and all works 100%. The only difference is that the lines in “Global Custom CSS” are not numbered.